Coffee club: Roasting processes in single beans and in bulk

Ezine

Published: Jan 7, 2014

Author: Steve Down

Channels: Base Peak

The coffee roasting process

I suspect that there is a lot of coffee being consumed this week as people try and reintegrate themselves into work after the long seasonal break. There certainly is in this office. Apart from the caffeine kick that coffee gives us, part of the attraction is the complex and characteristic smell of a freshly brewed cup and that derives completely from the roasting process.

Green coffee beans have very little flavour and a hay-like aroma but this changes dramatically when they are roasted. A cocktail of more than 850 volatile or semivolatile compounds has been identified to date in roasted coffee and the mixture varies according to the type of bean. Yet very little is known about the processes which occur within the bean when coffee beans are roasted and the quality of the final product relies strongly on the skill of the coffee roast master.

Scientists based in Germany and Switzerland have been investigating the coffee roasting mechanism and have just published their latest findings that were accomplished using mass spectrometry. However, they bypassed GC/MS, which is often chosen to analyse volatile organic compounds, in favour of the lesser used resonance-enhanced multiphoton ionisation time-of-flight mass spectrometry (REMPI-TOFMS). With this technique, aromatic compounds are ionised in favour of aliphatic compounds when the irradiating UV wavelength is chosen in the range 230-270 nm to be in resonance with the UV absorption bands of the aromatics. So, the important phenolic compounds in coffee can be selectively analysed.

Ralf Zimmermann, and colleagues from the University of Rostock, the German Research Centre for Environmental Health, Neuherberg, and Zurich University of Applied Sciences applied the high degree of selectivity and sensitivity of the REMPI-TOFMS to examine the aromatic compounds emitted during roasting at the macro and micro levels.

Batch roasting

REMPI-TOFMS has been used in the past to study industrial process gases and now the team applied it to the roasting gases produced in an industrial coffee roaster. The gases were directed via a heated probe to the ion source where they were irradiated with 266 nm pulses for REMPI-TOFMS.

The mass spectra were similar to those reported from small-scale coffee roasters. They were characterised by a series of peaks corresponding to the loss of a single electron from the parent molecules and featured many phenolic compounds such as the guaiacol derivatives and chlorogenic acids.

The method also allowed the time-intensity profiles of individual compounds to be measured as the roasting temperature was gradually increased. This implies that REMPI-TOFMS could be used for online process control during roasting on the industrial scale, as had been proposed by the researchers following studies with a small roaster.

Single bean surprises

In an accompanying set of pioneering experiments, the gases from single coffee beans were also analysed after heating with a hot air gun to simulate roasting. Using a specially designed microprobe, the gases were sampled from the surface of a single bean or from inside the bean. Coffea arabica beans from Bolivia and Coffea canephora beans from India were tested.

In this context, the single bean can be regarded as the smallest processing unit in coffee roasting. The ability to link processes at the elementary level, with about 150 mg per roasting, with those at the batch scale involving up to 180 kg of green beans, should lead to a better understanding of the fundamental processes taking place. The team focused on the volatiles that were produced from the pyrolysis or hydrolysis of the characteristic chlorogenic acids found in coffee and the shift from bulk to single bean produced some surprising results.

The time-intensity profiles of the volatiles from inside and outside the bean were different due to the time taken to diffuse through the cell walls of the bean. A second potential factor was the relative dissolution and partitioning of the compounds in the coffee. Closer study of these profiles for individual compounds revealed the breakdown pathways of the chlorogenic acids during roasting and identified two distinct reaction pathways.

A further fascinating revelation was the fact that the two species of coffee beans gave different volatile profiles, principally by the appearance of peaks appearing at m/z values higher than 200, which remain unidentified at present. However, it means that Arabica and Robusta coffee can be distinguished by analysing a single bean of each type.

So, REMPI-TOFMS can be used to study the chemical processes that take place when green coffee beans are roasted. It can also be used to monitor the roasting process of coffee beans in bulk and might eventually lead to a technique that could match the skill of the coffee roast master.

One potential problem with this concept is the high cost of the instrumentation for process control but the researchers argued that it could still be worthwhile given the high cost of coffee. A future goal should be to optimise the monitoring process before designing customised instruments that could reduce the overall cost.

The ultimate aim of the team is "linking these insights on single beans to processes occurring in industrial batch roasters and translate this to situations of industrial and commercial significance."